Method for aftertreatment of exhaust gases of internal combustion engines and corresponding device

ABSTRACT

A method and a device for the aftertreatment of exhaust gases of internal combustion engines, in particular for reducing the proportion of NO x  in the exhaust gases, the NO x  being reduced in the presence of O 2  using a reducing agent. The method and the device are characterized in that, prior to the reduction of the NO x , at least a proportion of the NO x  or NO contained in the exhaust gas is converted into NO 2 .

FIELD OF THE INVENTION

The present invention relates to a method for the aftertreatment of exhaust gases of internal combustion engines, in particular for reducing the proportion of NO_(x) in the exhaust gases, NO_(x) being reduced in the presence of O₂ via an SCR (Selective Catalytic Reduction) catalytic converter using a reducing agent. Furthermore, the present invention relates to a device for the aftertreatment of exhaust gases of internal combustion engines, in particular for executing the above-mentioned method.

BACKGROUND INFORMATION

Conventional systems for aftertreatment of exhaust gases are discussed in “Selective Catalytic Reduction,” Dieselnet Technology Guide, August 2000 (2000-06), pages 1-12, XPOO2163384, pages 1-11. According to this article, the oxidizing converter is used for producing NO₂ and for removing hydrocarbons from the exhaust gas of the internal combustion engine prior to the exhaust gases entering the SCR catalytic converter. According to this article, vaporized ammonia or an aqueous ammonia solution is to be introduced into the exhaust tract as a reducing agent.

Another system for exhaust emission control using an SCR catalytic converter is discussed in German Patent No. 100 21 693, in which the exhaust gases are activated prior to entering the SCR catalytic converter using non-thermal gas discharge plasma. According to this system, the hydrocarbons contained in the exhaust gas are initially absorbed to the greatest possible extent before the exhaust gas is exposed to the non-thermal gas discharge plasma. An ammonia-based reducing agent is then added to the exhaust gas pre-treated in this manner for the selective catalytic reduction of nitrogen oxides in the downstream SCR catalytic converter.

U.S. Pat. No. 5,893,267 discusses an exhaust emission control system in which a plasma reactor is arranged upstream from an SCR catalytic converter. In addition to exhaust gas, fuel is also supplied to the plasma reactor. According to this patent, the plasma oxidizes NO to NO₂ in a first step and breaks down the original hydrocarbons of the fuel into smaller molecules. In a second step, the smaller molecules should significantly increase the activity of the SCR catalytic converter. This should result in a significant synergy effect through the combination of the plasma with the SCR catalytic converter. Compared to using ammonia as a reducing agent, the use of hydrocarbons as a reducing agent is presented in this publication as being more practical, more cost-effective, and environmentally friendlier.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method and a device which make effective NO_(x) conversion possible without the use of ammonia as a reducing agent.

This object may be achieved by a method in which fuel as a reducing agent is introduced downstream from the oxidizing converter and upstream from the SCR catalytic converter.

In addition, this object may be achieved by using a system in which fuel as a reducing agent is supplied via an introduction device.

The present invention provides a method and a device for the aftertreatment of exhaust gases of internal combustion engines, in particular for reducing the proportion of NO_(x) in the exhaust gases, the NO_(x) being reduced in the presence of O₂ and a reducing agent. Such methods are referred to by the term SCR (Selective Catalytic Reduction) technology.

The method according to the present invention is based on the recognition that in SCR technology, when reducing agents are used which have no high selectivity, NO₂ is reduced while NO is only marginally influenced or not at all. Therefore, according to the present invention, the exhaust gases are pre-conditioned prior to reduction via SCR technology in such a manner that the largest possible proportion of NO_(x) or NO is converted into NO₂. Subsequent to the conversion of NO into NO₂, the proportion of NO in the exhaust gases is consequently lower and the proportion of NO₂ is correspondingly higher. Since NO₂ is converted in the subsequent reduction, the proportion of NO_(x) in the exhaust gases is reduced overall due to the prior conversion of NO into NO₂.

Fuel is used as a reducing agent in the present invention. Using fuel provides the advantage that fuel is available in the vehicle for supplying the internal combustion engine. Despite the relatively low selectivity of the fuel as a reducing agent, the proportion of NO₂ may be reduced using SCR technology due to the conversion of a large proportion of NO₂ of the exhaust gases.

NO_(x) or NO is converted into NO₂ using the oxidizing converter before the exhaust gases are supplied to the SCR catalytic converter. This has the advantage that fuel as a reducing agent having low selectivity may also be used for the aftertreatment of the exhaust gases and, in particular, for reducing the proportion of NO_(x) in the exhaust gases.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows an exhaust gas tract 10 in which exhaust gases A, coming from the internal combustion engine, are initially supplied to an oxidizing converter 12. Oxidizing converter 12 is used for converting NO, contained in exhaust gas A, into NO₂. Due to oxidizing converter 12, the proportion of NO in exhaust gas A is consequently reduced and the proportion of NO₂ in exhaust gas A is correspondingly increased. An introduction device 14, which introduces reducing agent into exhaust tract 10 in the form of, for example, fuel K, is arranged downstream from oxidizing converter 12. An SCR catalytic converter 16, which reduces NO_(x) in the presence of O₂ using reducing agent K, is arranged downstream from introduction device 14.

Using fuel provides the advantage that fuel is available in the vehicle. A different reducing agent, ammonia for example, must be stocked up separately as another operating supply item.

Although fuel as a reducing agent in SCR catalytic converter 16 has low selectivity, it has been found that fuel reduces NO₂. Due to the definite increase of the proportion of NO₂ in the exhaust gas due to the utilization of oxidizing converter 12, the overall proportion of NO_(x) is definitely reduced due to the large proportion of NO₂. In the ideal case, oxidizing converter 12 oxidizes the entire NO to NO₂. This would result in a largely complete reduction of the proportion of NO_(x) in the SCR catalytic converter using fuel as a reducing agent.

Using fuel as a reducing agent without utilizing oxidizing converter 12 would result in only the relatively small proportion of NO₂ in exhaust gas A being reduced. Due to low selectivity of the fuel, only a small proportion of NO is converted. According to the present invention, the proportion of NO₂ of the NO_(x) emission is consequently increased prior to the addition of fuel, to finally achieve an overall improved NO_(x) conversion due to the good NO₂ conversion using fuel as a reducing agent.

The proportion of platinum in the SCR catalytic converter should be as low as possible to prevent a conversion of NO_(x) into N₂O to the greatest possible extent. 

1-7. (canceled)
 8. A method for aftertreatment of an exhaust gas of an internal combustion engine, for reducing a proportion of NO_(x) in the exhaust gas, comprising: introducing fuel as a reducing agent into an exhaust gas tract downstream from an oxidizing converter and upstream from an SCR catalytic converter; converting at least a part of at least one of NO_(x) and NO contained in the exhaust gas into NO₂, wherein the conversion involves catalytic oxidation in the oxidizing converter arranged upstream from the SCR catalytic converter; and reducing, subsequent to the conversion step, NO_(x) in the presence Of O₂ by the SCR catalytic converter using the reducing agent.
 9. A device for aftertreatment of an exhaust gas of an internal combustion engine, for reducing a proportion of NO_(x) in the exhaust gas, comprising: an exhaust gas tract including an SCR catalytic converter; an oxidizing converter arranged upstream from the SCR catalytic converter; and an introduction arrangement for introducing fuel as a reducing agent into the exhaust gas tract between the oxidizing converter and the SCR catalytic converter, downstream from the oxidizing converter and upstream from the SCR catalytic converter; wherein at least a part of at least one of NO_(x) and NO contained in the exhaust gas is converted into NO₂, the conversion involving catalytic oxidation in the oxidizing converter arranged upstream from the SCR catalytic converter, and wherein, subsequent to the conversion, NO_(x) is reduced in the presence of O₂ by the SCR catalytic converter using the reducing agent. 